Method and apparatus for generating third harmonic signals

ABSTRACT

This invention relates to a method and apparatus for obtaining multiple resonant cavities that are simultaneously resonant at a fundamental frequency and its third harmonic frequency and to a metal plate beam splitter used in a resonator to separate the fundamental signal from the third harmonic signal. The beam splitter comprises a series of plates made of metal or metallized dielectric; the dielectric being plated to obtain high surface conductivity. The plates are dimensioned so as to give the desired attenuation of the fundamental frequency and are spaced relative to each other in such a manner as to mask the transverse area occupied by the field of the resonator. The beam splitter is inserted into the interaction region of a doubly resonant FabryPerot cavity or other cavity resonator and may be used in the microwave through submillimeter region.

United States Patent Bechtold etal.

[54] METHOD AND APPARATUS FOR GENERATING THIRD HARMONIC SIGNALS Inventors: George W. Bechtold, Tucker, Ga.; Vernon E. Derr, Boulder, Colo.; James C. Wiltse, Orlando, Fla.

The United States of America as represented by the Secretary of the Army Filed: Jan. 26, 1971 Appl. No.5 109,917

Related US. Application Data Continuation-impart of Ser. No. 748,465, Jul 29, 1968, abandoned.

Assignee:

US. Cl. ..307/88.3, 321/69 R, 333/83 R Int. Cl....'. ..H02m 5/06 Field of Search ..321/69; 307/883 [15] 3,699,353 [4 Oct. 17, 1972 Primary Examiner-Roy Lake Assistant Examiner-Darwin R. Hostetter Attorney-Harry M. Saragovitz, Edward J. Kelly, Herbert Ber] and Milton W. Lee v [57] ABSTRACT This invention relates to a method and apparatus for obtaining multiple resonant cavities that are simultaneously resonant at a fundamental frequency and its third harmonic frequency and to a metal plate beam splitter used in a resonator to separate the fundamental signal from the third harmonic signal. The beam splitter comprises a series of plates made of metal or metallized dielectric; the dielectric being plated to obtain high surface conductivity. The plates are dimensioned so as to give the desired attenuation of the fundamental frequency and are spaced relative to each other in such a manner as to mask the transverse area occupied by the field of the resonator. The beam.

splitter is inserted into the interaction region of a doubly resonant Fabry-Perot cavity or other cavity resonator and may be used in the microwave through submillimeter region.

6 Claims, 3 Drawing Figures PATENTEUBLT 17 I812 3,699,353

FIG. 3

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IN VE N TORS,

GEORGE w. BECHTOLD VERNON E. DERR & JAMES c. W/LTSE.

' METHOD AND APPARATUS FOR GENERATING THIRD HARMONIC SIGNALS BACKGROUND OF THE INVENTION particular, to a microwave interferometer technique wherein a third harmonic signal is generated. Various types of optical and microwave interferometers have been constructed in the past including the Michelson and the Fabry-Perot interferometers that employ, perhaps, the two most widely known interferometer techniques. It is to the Fabry-Perot interferometer technique thatthe invention disclosed here is directed.

More specifically, the instant invention is directed to the interaction region of a pair of Fabry-Perot cavities and particularly to the method of producing a third harmonic signal and to the beam divider or beam splitter used in such cavities resonators. Unlike the ordinary interferometer, however, the instant invention employs a novel beam splitter in a totally resonant gaseous environment therebytaking advantage of the multiple quantum effect, whereby the energy level transition of several photons at the fundamental frequency for each photon transition at the harmonic frequency generates the third harmonic signal from the impressed fundamental frequency signal. To obtain high efficiency it is necessary that a pair of Fabry-Perot-cavities be simul-- frequencies. The multiple quantum effect can occur in any gas with suitable energy level configurations and allowed transitions. The'requirement is that there be three energy levels with separations between pairs of levels which add up'to the total photon energy. Gaseous thionyl fluoride, methylene chloride and nitrosyl fluoride are suitable gases for multiple quantum transitions but in addition, many other gases have suitable energy level configurations. Although the instant invention is not concerned with the application of an external magnetic or electric field, such techniques could clearly be employed to modulate the output of the instant invention.

Devices other than interferometers, and including reentrant cavities, waveguides, and coaxial resonators, have been used in'the past to generate high frequency millimeter and submillimeter signals; however, none of these devices appear well suited for use at higher frequencies. Thionyl fluoride, methylene chloride and nitrosyl fluoride have suitable energy levels in the millim eter and submillimeter region where interferometers are the most efficient resonators.

SUMMARY OF THE INVENTION The method and device of the instant invention is generally directed to a high efficiency microwave generator employing a beam splitter in the interaction region of a pair of cavities in such a manner as to produce a multiple resonant F abry-Perot cavity that is Simultaneously resonant at a fundamental and its third harmonic frequency.

It is the general purpose of the instant invention to produce a resonator structure employing interferometric techniques thereby giving: especially high Q values, hence low losses; a high filling factor (essentially an cfficiency factor relating to the efficiency of the device in generating third harmonic frequencies); and a large in teraction volume thereby. simplifying coupling and mode discrimination. These particular objectives are accomplished generally, by placing a series of metal strips or metallized dielectric strips in the interaction region of the instant doubly resonant Fabry-Perot microwave cavity in such a manner that the fundamental frequency signalfrom a microwave horn radiator is BRIEF DESCRIPTION OF THE DRAWING The exact nature of this invention will be readily apparent from consideration of the following specification relating to the annexed drawing in which;

FIG. 1 shows the preferred embodiment of the instant invention by showing a doubly resonant interferometric device with a beam splitter plate inserted in the interaction region.

FIG. 2 is a view of the beam splitter of the instant invention.

FIG. 3 is a perspective view of the beam splitter of the instant invention.

DESCRIPTION OFTHE PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown the beam splitter 3 employed in the interaction region of doubly resonant Fabry-Perot cavity 10. A microwave through submillimeter signal of frequency f, is emitted from transmitting aperture 2 and is coupled to the resonant cavity by partially transmissive plate 6 containing varithrough the beam splitter 3 (either directly or after reflection from reflector plate 4) and to coupling plate 5 thence into receiving aperture 7 where it, the third harmonic signal, is received and utilized as desired.

' Mechanical tuning of plate 6 to resonate frequency f FIG. 2 shows a view of beam splitter 3 from above FIG. 1 and toward aperture 2. It is seen here that the beam splitter 3 is essentially composed of spaced larninations placed in the interferometer with the E field parallel to the strips or laminaations. FIG. 3 shows more clearly the spaced laminations of beam splitter 3 with the desired or preferred dimensions. The metallic strips act as a waveguide beyond cutoff for signals below the cutoff frequency when placed such that the E field is parallel to the strips as indicated above. The

spacing a between the strips is calculated from the relation 2a=)t where A is the wavelength of the cutoff frequency. For a cutoff fundamental frequency of 60 GHz for example, the spacing a is approximately 0.0987 inches or approximately one-fourth centimeter. Dimensions and d are selected so that they are large enough to mask the transverse area occupied by the field, and were approximately inches each in the device constructed, while the dimension b is selected as to be large enough to give the required attenuation of f and was approximately 0.5 inches in the device constructed.

To obtain the necessary high Q and low transmission loss for the higher frequency signal, it is necessary that the strips of beam splitter 3 be thin (approximately 0.003-0.005 inches) of high conductivity, and have a smooth finish. These plate fabrication techniques increase the Q of the device through decreasing diffraction and attenuation losses. Beam splitter 3 is preferably constructed of gold plated brass strips; attached at each end to small anchor blocks. Tension required to maintain the strips parallel is applied to the strips by screws that are threaded through the mounting frame and anchor blocks. The anchor blocks are silver soldered to the strips to prevent cold flowing. Small plastic spacers were placed near the ends of the strips to maintain parallel spacing between strips. As an alternate, metallized dielectric may also be employed for strips with predictably good results.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or alterations may be made therein without departing from the spirit or scope of the invention as set forth in the appended claims.

We claim:

1. A method of generating a third harmonic frequency and simultaneously resonating said harmonic frequency and the fundamental frequency in a doubly resonant Fabry-Perot cavity comprising the steps of:

generating a third harmonic frequency by the multiple quantum effect in a suitable gas located in said cavity by introducing a fundamental frequency therein,

angularly orienting a beamsplitter comprised of a plurality of thin, closely spaced plates of high surface conductivity in the interaction region of the interferometer cavities,

'4 tuning each of the cavities to resonate the fundamental frequency and third harmonic frequency simultaneously; and

extracting the third harmonic frequency from its cavity as an output signal.

2. A doubly resonant Fabry-Perot cavity having a resonant gas therein for generating a third harmonic signal in the microwave through submillimeter region by the multiple quantum effect, comprising:

a reflector and a first partially transmissive plate parallel thereto,

a second partially transmissive plate angularly oriented with respect to said reflector and said first partially transmissive plate,

a beamsplitter angularly positioned in, and at least partially masking, the interaction regionof said doubl resonant Fabr erot cavit created b said rflector and said fii'st and sec ond partially transmissive plates, said beamsplitter further comprising a plurality of thin parallel spaced plates of high conductivity for separating the fundamental frequency from the third fundamental frequency in said doubly resonant cavity, whereby the fundamental frequency resonates between said reflector and one of said partially transmissive plates and the third harmonic resonates between said reflector and the other of said partially transmissive plates.

3. A doubly resonant cavity according to claim 2, and further comprising:

a first coupling means for introducing said fundamental signal into said interaction region; and

a second coupling means for withdrawing said third harmonic signal from said interaction region.

4. A doubly resonant cavity simultaneously resonant at the fundamental and its third harmonic frequency according to claim 2, wherein:

said plates are spaced relative to each other in such a manner as to act as a waveguide beyond cutoff to the fundamental frequency signal.

5. A double resonant cavity according to claim 10 wherein said spaced plates are dimensioned to substantially mask said interaction region of said cavity.

6. The doubly resonant cavity as in claim 5 wherein the resonant gas is selected from the group consisting of thionyl fluoride, methylene chloride and nitrosyl fluoride. 

1. A method of generating a third harmonic frequency and simultaneously resonating said harmonic frequency and the fundamental frequency in a doubly resonant Fabry-Perot cavity comprising the steps of: generating a third harmonic frequency by the multiple quantum effect in a suitable gas located in said cavity by introducing a fundamental frequency therein, angularly orienting a beamsplitter comprised of a plurality of thin, closely spaced plates of high surface conductivity in the interaction region of the interferometer cavities, tuning each of the cavities to resonate the fundamental frequency and third harmonic frequency simultaneously, and extracting the third harmonic frequency from its cavity as an output signal.
 2. A doubly resonant Fabry-Perot cavity having a resonant gas therein for generating a third harmonic signal in the microwave through submillimeter region by the multiple quantum effect, comprising: a reflector and a first partially transmissive plate parallel thereto, a second partially transmissive plate angularly oriented with respect to said reflector and said first partially transmissive plate, a beamsplitter angularly positioned in, and at least partially masking, the interaction region of said doubly resonant Fabry-Perot cavity created by said reflector and said first and second partially transmissive plates, said beamsplitter further comprising a plurality of thin parallel spaced plates of high conductivity for separating the fundamental frequency from the third fundamental frequency in said doubly resonant cavity, whereby the fundamental frequency resonates between said reflector and one of said partially transmissive plates and the third harmonic resonates between said reflector and the other of said partially transmissive plates.
 3. A doubly resonant cavity according to claim 2, and further comprising: a first coupling means for introducing said fundamental signal into said interaction region; and a second coupling means for withdrawing said third harmonic signal from said interaction region.
 4. A doubly resonant cavity simultaneously resonant at the fundamental and its third harmonic frequency according to claim 2, wherein: said plates are spaced relative to each other in such a manner as to act as a waveguide beyond cutoff to the fundamental frequency signal.
 5. A doubly resonant interferometer according to claim 2 wherein said spaced plates are rectangular in shape and are dimensioned to substantially mask said interaction region of the interferometer.
 6. The doubly resonant cavity as in claim 5 wherein the resonant gas is selected from the group consisting of thionyl fluoride, methylene chloride and nitrosyl fluoride. 